on p. 16 we read:
"In Igbo, for instance, the underdotted i, o and u characters never have a superimposed accent"
but that is plainly false. the author may have formed such impression from the admitted fact that tonemarking is rare in igbo texts. however, provision for tonemarking has been part of every known igbo orthography (except for the solipsistic spelling found in echeruo's "igbo-english dictionary", yale university press 1998).
the rarity of tonemarked igbo reflects multiple factors:
(1) the relatively low functional load of tone in distinguishing lexical items in igbo, compared to some other nearby languages;
(2) the practical failure of language policy after the biafran war as part of the general collapse of schools and universities in nigeria;
(3) user disinterest in igbo orthography overall, due to factors (1) and (2) above as well as to broader sociological conditions including the traditional political fragmentation of igbo-speaking area and its extreme natural dialect diversity exacerbated by missionary rivalry and mischief both before and after the war.
igbo has two alternative tonemarking conventions, but whichever one is employed, the fact is that ALL vowels, dotted or otherwise, upper and lower case, as well as the bare nasals m/M and n/N, can appear with superposed tone marks, as follows:
(i) ibadan convention, also known as williamson/emenanjo. convention:
grave accent (= every syllable bearing L tone)
macron (= first H tone after a downstep juncture, popularly but inaccurately called "drop/mid tone")
(ii) welmers/nwachukwu convention:
grave accent (= first syllable of a sequence of L tones)
acute accent (= first syllable of a sequence of H tones, including such a sequence which follows a downstep juncture).
in convention (i), every unmarked syllable is understood to bear a H tone which is not downstepped with respect to the preceding syllable. in convention (ii), every unmarked syllable is understood to bear the SAME pitch level as the preceding syllable, whether that syllable is L or H.
of the two conventions, (i) is better known, but tends to confuse any user who wants to mark downstep. lexical downsteps are few, and so the issue of downstep is more important in phrases, but this is not trivial because most personal and place names show phrasal tones. another problem with convention (i) is the ease with which handwritten macrons can be confused with grave accents. but if downstep is ignored, convention (i) is adequate although it suffers from a further problem, that it leaves the reader in doubt about the tone of an isolated word which appears without tonemarks: whether the tone of such a word is all H or if the writer simply chose not to mark and the word could have any tone whatsoever. none of these confusions is possible with convention (ii).
as a minor correction i should also point out that the geographic positions of igbo and e.do on the coverpage map have been mistakenly reversed. i realise that the map is not intended to be precise, but even in terms of RELATIVE position, it's wrong to place the e.do-speaking area to the EAST of igbo, because it is located entirely to the WEST.